These are soils in which the sodium content or Na saturation is greater than 15%. That is, if the exchange sites of the soil is occupied by sodium to the tune of 15%. The term “exchangeable sodium percentage” (ESP) or sodium adsorption ratio (SAR) is usually used and expressed as
ESP = ex Na meg/100g soil x 100
C.E.C. meg/100g soil
For example, (i) if C.E.C. of a soil is 10meq/100g in which Mg = 2.5, Ca = 2.0, K =1.0
Na = 2.5 and A1 = 2.0 meq/100g soil
Then % Na = 2.5 meq/100g x 100
10meq/100g
= 25% Na
(ii) if Na = 1.0meq/100g soil then % Na = 1.0 x 100
10
= 10% exchangeable sodium
Conclusion: Soil (i) is alkaline whereas solid (ii) is not.
Saline-Alkaline Soils
A soil which is both saline (electrical conductivity > 4mmhos/cm) and alkaline (ESP > 15%) is referred to as saline-alkali soil or saline-sodic soil.
Such a soil has pH value of less than 8.5: and if the salts are leached out, excess sodium reacts with carbon dioxide to form sodium carbonate and the pH of the soil may be as high as 9 or 10, thus turning the soil to pure sodic soil.
Read Also: Sources of Salt-Affected Soils
